That is narrowing now. HD Masters are the trophies for these studios. For many titles (especially those currently being filmed in the HD format), then will NOT get any better, meaning they are at the pinnacle. It's far riskier today for the studios to play around with their pure, raw, 100% perfect product that, if a mistake is made and it stumbles down the wrong path, it can legitmately be lost forever.
Down the road we'll see more advanced filming/displaying of movies, but we're not talking about what hasn't been made yet.